Background: Lymphopenia (LMP) may lead to worse outcomes for patients with glioblastoma (GBM). This study is a secondary analysis of the CCTG CE.6 trial evaluating the impact of chemotherapy and radiation on LMP, as well as the association of LMP with overall survival. Methods: CCTG clinical trial CE.6 randomized elderly GBM patients (≥ 65 yrs) to short course radiation alone (RT) or short course radiation with temozolomide (RT + TMZ). In this study LMP (mild-mod: grade 1-2; severe: grade 3-4) was defined per CTCAE v3.0 criteria, and measured at baseline, 1 wk and 4 wks post-RT. Pre-selected key factors for the analysis included age, sex, ECOG, extent of resection, MGMT methylation, MMSE, and steroid use. Multinomial logistic regression models were used to identify factors associated with LMP and multivariable Cox regression models were used to study effect of LMP on survival. Results: A total of 562 patients were included for analysis (281 RT vs 281 RT+TMZ). At baseline, both arms (RT vs RT+TMZ) had similar rates of mild-mod (21.4% vs 21.4%) and severe (3.2% vs 2.9%) LMP. The 1 wk post-RT LMP rates were also similar (p = 0.25). However, RT+TMZ pts were more likely to develop both mild-mod LMP (18.2% vs 27.9%) and severe LMP (1.8% vs 9.3%) [p < 0.001] at 4 wks post-RT. Developing mild-mod and severe LMP post-RT were both associated with baseline LMP (p < 0.001) and RT+TMZ (p < 0.001). Severe LMP at 4 wks post-RT was also associated with biopsy only (p < 0.02). After adjusting for confounding factors, 4 wks post-RT LMP was not significantly associated with PFS or OS regardless of severity. However, baseline LMP (HR 1.3) was significantly associated with worse OS (HR: 1.30, 95% C.I.: 1.05-1.62, p = 0.02), regardless of MGMT status. Other factors significantly associated with worse outcome included: males (HR 1.41), biopsy only (HR 1.59), and lower MMSE (HR 1.03). Conclusions: Short course RT alone does not lead to LMP after treatment. Development of LMP post-RT is associated with addition of TMZ and baseline LMP. However, only baseline LMP is associated with worse OS regardless of MGMT status. This may be considered as a prognostic biomarker for elderly GBM patients and warrants further validation. Clinical trial information: NCT00482677.